When photographs are taken of people using a digital or traditional film camera with a flash, a common artifact that can appear in the picture is referred to as "red-eye." This artifact manifests itself as a red coloration within the pupils of eyes, instead of the natural black or dark gray color, and is referred to as a "flash effect."
Red-eye is caused by the reflection of light from a flash off of the blood vessels of the retinas of a subject's eyes. One can try to reduce the red-eye by increasing the light level in the room by turning on all the lights. Alternatively, one can have a subject look at a bright light, such as a room lamp, just before taking the flash picture. Both methods of red-eye reduction described above cause the subject's pupils to contract, thereby reducing the amount of reflective surface.
Interestingly, this flash effect appears as red eyes in humans, but as yellow eyes in dogs and cats, and green eyes in some other animals. Photographers and image compositors commonly touch up their images and remove the flash effects from the eyes.
Adobe's PhotoDeluxe.TM. 2.0 and Microsoft's PictureIt!.TM. 2.0 are digital photo-editing software applications that offer semi-automatic red-eye removal as a feature. Such software applications require a user to select a region surrounding a group of pixels that are corrupted by red-eye. In operating PhotoDeluxe 2.0, for example, a user selects a red-eye area, clicks a button and then the eyes return to their natural color.